1. Field of the Invention
The present invention relates to an idling air flow rate adjusting device for an internal combustion engine.
2. Description of the Prior Art
Such an adjusting device is known which includes a by-pass passage by-passing a throttle valve arranged within an intake air passage, and a variable orifice means within the by-pass passage whose opening determines the idling air flow rate. The orifice means is so adjusted that, when the throttle valve is substantially fully closed, an optimum amount of idling air is supplied to that portion of the intake air passage which is downstream of the throttle valve and then to the engine, thus permitting a stable idling operation of the engine.
In an internal combustion engine, such as an EGI-type engine provided with an electronically controlled gasoline injection valve, or an engine in which liquid propane gas is used as fuel, blow-by gas produced in the engine body is normally recirculated from a crank case to that portion of the intake air passage which is downstream of the throttle valve and of the connection between the intake air passage and the by-pass passage. For this purpose, there is provided a blow-by gas recirculation passage which includes a control valve. When, however, the amount of blow-by gas produced in the engine body exceeds the capacity of the control valve, the excessive amount of the blow-by gas is recirculated through an auxiliary passage to an air cleaner or to that portion of the intake air passage which is upstream of the throttle valve and of the connection between the intake air passage and the by-pass passage. In such a case, the by-pass passage is supplied, besides fresh ambient air, with the blow-by gas.
Conventionally, the above-mentioned variable orifice means in the idling air flow rate adjusting device includes a valve element in the form of an adjusting screw having a tapered free end, and a valve seat therefor which is integral with, or forms part of by-pass passage wall.When the by-pass passage is supplied with the fresh ambient air and the excessive blow-by gas, dust contained in the air which could not be removed by the air cleaner, and oil particles in the blow-by gas tend to gradually accumulate on the surface of the tapered end of the valve element and of the valve seat. Such an accumulation results in reduction of the effective opening area of the orifice and hence, of the idling air flow rate, which is disadvantageous in that, with correspondingly reduced idling revolutional speed of the engine, the operation of the engine becomes unstable, and furthermore in that content of carbon oxide in the exhaust gas increases due to insufficient idling air flow rate for a given amount of fuel.